Often when capturing an image of an object using an image capturing apparatus, for example a camera, it is desirable to be able to change a zoom setting for the image. With the distance between the camera and the object remaining constant, a high zoom factor allows an image of the object to be captured at a high level of magnification and a narrow field of view. A low zoom factor allows an image of the object to be captured at a low level of magnification and a wide field of view. In the example of a camera the high zoom factor is characteristic of a telephoto lens setting and the low zoom factor is characteristic of a wide-angle lens setting.
Different zoom factors require different effective focal lengths of a zoom optical system. For a telephoto lens setting the effective focal length is relatively long and for a wide-angle lens setting the effective focal length is relatively short.
In order for images to be captured with different zoom settings it is preferable that the effective focal length of a zoom optical system of the camera must be variable whilst keeping the image of the object in focus. Such a zoom optical system may be constructed using at least two lenses which provide a variable focus.
A known zoom optical system comprises an array of solid lenses which lie along a common light path. By varying positions of these lenses along the light path, different effective focal lengths can be obtained whilst keeping the image in focus, thus allowing images having different zoom settings to be captured. Zoom optical systems of this type are however relatively bulky and mechanically complex. Movement of the different lenses may be performed either manually or automatically, but these methods are typically relatively expensive and lacking in robustness. The range of zoom factors provided by such a zoom optical system depends on parameters including the focal power of the individual lenses and the distances between the lenses along the light path which can be achieved. An increase in an upper limit of the zoom factor range of a zoom optical system of this type will generally increase the bulk and complexity of the zoom optical system.
Rather than one zoom optical system providing both a telephoto and a wide-angle zoom function, it is often necessary to swap the lenses of a camera between a telephoto and a wide-angle system in order to obtain a range of different zoom factors. This is a relatively slow and inconvenient process and requires a camera user to carry different lenses in addition to the camera.
International patent application WO 03/069380 describes a fluid meniscus lens. This lens comprises a fluid meniscus which separates a first fluid and a second fluid and which has a curvature. By varying this curvature it is possible to change the focal length of the lens and the focus of an image. In applications where the lens is to be used in various orientations, the fluids are preferably density matched to avoid unwanted gravitational effects. Therefore, two liquids, such as oil and water, which are density matched are used.
Two such fluid meniscus lenses could be incorporated in a zoom optical system in order to capture images having different zoom factors. To achieve a large range in zoom factor requires a large optical power change in each the fluid menisci. Since the refractive indices of the two liquids are not greatly different, and because the extent to which the curvature may be varied is limited, the optical power range of the fluid meniscus lenses is relatively small. This imposes a limit on the possible zoom factor range provided by the zoom optical system. In particular, one lens component in a zoom lens system (typically that closest the image capture device) requires the greatest optical power range during zooming, so the amount of zooming is constrained by the relatively limited optical power range of the fluid meniscus lens used as that one lens component.